As for the method of automatically changing the running speed of a vehicle wherein the output side of the engine is connected to a torque converter provided with a lock-up clutch, the output side of the torque converter is connected to the input side of a hydraulically actuated power transmission or speed change gear having a plurality of speed stage clutches. The output side of the power transmission is connected through a differential gear mechanism to the left and right drive wheels. An automatic speed change method has been known wherein speed increasing points and speed decreasing points, i.e., speed change points, are set on the basis of the relationship between the tractive force at each of the speed stages of the power transmission when the engine is developing a maximum speed and the vehicle speed at that time. The vehicle speed is detected from the number of revolutions of the engine and the like. When the thus detected vehicle speed exceeds a speed change point, an automatic speed changeover to a next speed stage can be made.
For example, as shown in FIG. 1, the arrangement is made such that speed change points f.sub.1 to f.sub.6 are set from the relationship between the tractive forces when the engine is developing a maximum output at forward first to seventh speeds F.sub.1 to F.sub.7 and the running speeds of the vehicle. When the vehicle speed exceeds each of the speed stages, it is automatically changed over to a next speed stage.
That is to say, the maximum outputs of the engines mounted on dump trucks for use as construction vehicles are not so high as those outputs required for them, and the weight of such vehicles in a loaded condition varies significantly from that in an unloaded condition. Further, because the loadage and the gradient of roads on which vehicles run vary so much, the loading imposed on dump trucks for use as construction vehicles fluctuates largely. Also since it is difficult to detect the cargo on board and the gradient of roads with a high accuracy, it has been a common practice for dump trucks for use as construction vehicles to set speed change points which correspond always to the maximum output condition developed by the engine mounted on each of them, assuming cases where the loading imposed on them is high, and to run them with their engines developing maximum outputs.
In the above-mentioned conventional automatic speed change method, in case the loading on a vehicle is extremely light as, for example, in the case of an unloaded vehicle running on a flatland or in case a loading on the vehicle is comparatively light as in the case of the vehicle in a light load condition running up a slope, the output developed by the engine is not utilized fully thus consuming an extra fuel ineffectively. This is because the output required for a dump truck for use as a construction vehicle in the above-mentioned light load condition is in fact less than the maximum output developed by the engine. Further, in mining slopes and so forth, a plurality of dump trucks for use as construction vehicles having different engine outputs usually run along the same course. So, it is not allowed for only one truck to run at a higher speed than the others, at a maximum output developed by its engine. Rather, the truck must be driven at a lower speed which is adjusted to the lower speeds of other vehicles. Therefore, vehicles with higher engine outputs are required to run at a lower speed stage; that is, at a higher number of revolutions of the engine, thereof, resulting in increases in the fuel consumption rate.
Whilst, as to the engine, output, there are cases where even when a vehicle is driven at a speed stage higher than the current running speed, the same tractive force can be obtained. In FIG. 1, if a vehicle subjected to a load, for example, at point "a" is running at a speed of V.sub.1, then the vehicle speed is set at the forward fifth speed F.sub.5, whilst from the viewpoint of the tractive force (engine output), it becomes possible for the vehicle to run at the speed stage for the forward sixth speed F.sub.6.
Whilst, in case a dump truck for use as a construction vehicle which is running on a flatland at a high speed stage runs up a slope, for example, or in case the running condition of the vehicle is changed over to an idling condition by reducing the degree of opening of the accelerator to zero, it is required for the vehicle to make a number of speed changes in turn from high speed stages to low speed changes in a short time. If such speed changes are made a plurality of times in a short time, then excessive forces tend to be exerted on the power transmission system from the transmission or speed change gear to the drive wheels thus impairing the durability of the power transmission system and causing shocks each time a speed change is made and also causing plural changes in the vehicle speed thereby giving the driver rough ride.
Further, as for the conventional speed change operation for the above-mentioned dump trucks for use as construction vehicles, there has been known a speed change operational method wherein a speed change is made in a condition that the lock-up clutch is disconnected and the output side of the engine is connected through a torque converter to the input side of the power transmission or speed change gear and, after the completion of the speed change, the lock-up clutch is connected again so that the output side of the engine may be connected directly to the input side of the power transmission.
In such a speed change operational method, when the vehicle is in a normal running condition, the engine output can be directly transmitted to the power transmission not through the intermediary of the torque converter thus reducing the power losses. Each time a speed change is made, the engine output is input or transmitted through the torque converter to the power transmission so that even if the revolving speed of the input shaft of the power transmission does not correspond to that of the output shaft, the shocks due to speed changes can be absorbed by the torque converter to some degree thereby reducing the magnitude of such shocks.
However, if after the completion of a speed change and when connecting or engaging the lock-up clutch, the number of revolutions of the engine does not correspond to that of the input shaft of the speed change gear, then a shock due to a speed change will occur.
When, for example, a speed change to a speed stage lower than the present speed stage, i.e., a shift-down operation is made, the load imposed on the engine will increase, thus causing a reduction in the number of revolutions of the engine so that when the lock-up clutch is connected the number of revolutions of the engine becomes lower than that of the input shaft of the power transmission or speed change gear thereby causing a shock due to the speed change.
Further, as for the conventional speed change operation, there has been known a speed change operational method wherein the supply of the pressurized fluid discharged by a pressurized fluid source to each of a plurality of speed stage clutches is controlled by each of the speed change valves associated therewith, and a progressive pressure increasing valve for gradually raising the fluid pressure in the pressurized fluid supply source is provided so that, when supplying the fluid under pressure to a predetermined speed stage clutch, the fluid pressure can be increased progressively to ensure smooth connection or engagement of the speed stage clutch without causing any shocks due to a speed change.
Each of the speed stage clutches comprises a plurality of clutch plates which are rendered operative when they are urged into contact with one another and which are rendered inoperative when they are separated from one another. If the time required until the fluid pressure is increased to a predetermined value becomes longer, then each of the speed stage clutches can be engaged more smoothly. However, on the other hand, a longer time is required to make a speed change. Whilst, if the fluid pressure is increased to a predetermined value in a short time, then the time required for the speed change can be reduced, but on the other hand the speed stage clutch is engaged quickly thus causing a big shock due to the speed change. For this reason, the time required for increasing the fluid pressure to a predetermined value is usually set at one intermediate value of the two; that is to say, progressive pressure increasing characteristics are set by means of a progressive pressure increasing value.
Whilst the loading imposed on the dump trucks for use as construction vehicles varies remarkably between loaded and unloaded conditions, and so they are driven repeatedly in heavy and light load conditions. When the vehicle is running in a heavy load condition, a high load is applied to the power transmission so that the torque transmitted thereby becomes high, whilst when running in a light load condition, a low load is applied to the transmission so that the torque transmitted thereby becomes low.
If the torque transmitted to the power transmission is high, the clutch plates tend to slip, while, if the torque is low the tendency to causing slipping of the clutch plates is reduced. Therefore, if as mentioned above, the time required for increasing the fluid pressure to a predetermined value is constant and if the torque transmitted to the power transmission is high, the time required until the clutch plates are engaged completely without causing any slip to ensure satisfactory power transmission tends to increase thus increasing the time required for making a speed change. Whilst, in case the torque transmitted to the power transmission is low, it becomes possible to attain a condition that satisfactory power transmission can be made in a short time thereby increasing the tendency of occurrence of big shocks due to a speed change.